A friend's got a 89 sport nautique with a Ford PCM 351 engine with Holly carb. While trying to accelerate the boat starts slipping and reducing speed, cutting out and acts inconsistent, he's tried different gas stations, using gas cleaner &/or water removal from the gas tank to try and troubleshoot, but nothing's helping. Also he replaced all of the spark plugs still cutting out and some backfiring type noises persist. We think it might be the carb. but not sure... Anyone have similar problems? Any suggestions?

Holley's can't take backfiring too well. They have a proportioning valve that has a tendancy to blow out if it backfires thru the carb. Was the backfire out the carb or exhaust? If you end up replacing the carb stay away from Holley. Ask any gearhead.

Friend with the exact boat/motor was having similar problems. Problems were all electrical. Redid the grounds and cleaned up/redid all the connections at the ignition switch. Seems to have solved all the problems.

I had a '90 prostar 190 with same engine. I was having the same problem you're describing. It was almost like the boat was running out of gas. My mechanic replaced the power valve and the problem was solved. He suggested doing this before rebuilding or replacing the carb. It was the best 30 bucks I ever spent on that boat.

Sounds electrical? Check all the connections under dash......If boat starts to hesitate under way you definitely want to get the timing gun on the drive belts. Also, check all connections to ignition and distributor points(wire brush will clean nicely).... Good luck and let us know.....

OK, you have way too much advise here. Best to cover all the basics as any mechanic would do to diagnose you problem.

1.Check compression to make sure there are no internal engine problems. 2. Check for spark and see that plugs are clean and unfouled. Run the motor in pitch darkness to see if plug wires are arc-ing. And see that you have sufficient voltage to the distributor. Check for loose or cracked cap and look at the rotor. If everything checks-out, measure timing.

3. Try a "known good" carb. For me this is easiest because I have multiple Chevy 350's with tons of spare parts. If you have to, buy a carb (most are ready to run out of the box.) Knowing full-well you will return it if it doesn't help.

Sandy is onto something here. I would definetly look at the ignition system, but take it one step farther and scope or at the least ohm the ignition wires. Take a good hard look at the distributor cap if it has one. Any kind of carbon tracking is an idication of a missfire. If it has old school points replace them and the condenser. Is this a DIS motor? Distributerless that is.

As Sandy also said the Holley carbs are prone to destroy power valves when a motor backfires. Please do not change the carb or you will cause all kinds of problems. If it has a problem fix the problem or rebuild it, but fuel mixture is too important on a boat motor. The carb is most likely not the problem as you said it was intermitant. Fuel problems are usually more consistant under load.